concat_idents can create empty identifiers

[ExpHP]

I posted this casually on the tracking issue some months back:

An amusing fact about the current implementation of concat_idents! is that it will accept an empty
argument list, making it possible to construct the empty string as an identifier:

error[E0425]: cannot find value `` in this scope
 --> src/main.rs:5:5
  |
5 |     concat_idents!();
  |     ^^^^^^^^^^^^^^^^^ not found in this scope

(good luck actually doing anything with it)

I left it at that because I never thought there would be any serious discussion of stabilizing the feature. Never say never, huh?

This is an easy fix and I am working on a PR. I just made this issue so that I have a number to name the test case after. 🦆

[ExpHP]

Random aside: I thought I would try to do the same thing with proc_macro::Term. Apparently, there is a sanity check, at least in some places:

error: expected identifier, found reserved identifier ``
 --> test.rs:6:1
  |
6 | prox_max::to_the_max!{}
  | ^^^^^^^^^^^^^^^^^^^^^^^ expected identifier, found reserved identifier

error: expected expression, found reserved identifier ``
 --> test.rs:6:1
  |
6 | prox_max::to_the_max!{}
  | ^^^^^^^^^^^^^^^^^^^^^^^ expected expression

(the macro generated this, where ◌ is Term::new("", span))

#[inline(never)]
#[no_mangle]
fn ◌() {
    println!("hi there!");
}

fn main() {
    ◌();
}

---

Edit: Hm. _ is also on the reserved identifiers list. I wonder...
Edit: ...nope. ◌ is the only reserved identifier that can be constructed by concat_idents!.

[ExpHP]

Tʜᴇ Pʟᴏᴛ Tʜɪᴄᴋᴇɴs

You can construct regular keywords as identifiers too:

error[E0425]: cannot find value `fn` in this scope
 --> src/main.rs:3:13
  |
3 |     let x = concat_idents!(fn);
  |             ^^^^^^^^^^^^^^^^^^ not found in this scope

but it does not appear that you can do Unforsakeable Things with this because you can't use the macro in patterns for some reason.

#![feature(concat_idents)]
fn main() {
    let concat_idents!(fn) = 3; // ERROR non-pattern macro in pattern position
    let x = concat_idents!(fn);
}

Obviously my planned simple fix of simply disallowing an empty list would not fix this issue... but is it even an issue? I'm not sure.

Edit: This is not an issue I think, because raw identifiers will enable you to do the same